Risperidone (also known as 4-[2-[4-(6-fluorobenzo[d]isoxazol-3-yl)-1-piperidyl]ethyl]-3-methyl-2,6-diazabicyclo[4.4.0]deca-1,3-dien-5-one and marketed under the trade name RISPERDAL®) is an atypical antipsychotic medication. The chemical structure of risperidone is shown in formula (1).

Risperidone is most often used to treat delusional psychosis (including schizophrenia), but risperidone is also used to treat some forms of bipolar disorder, psychotic depression, obsessive-compulsion disorder, and Tourette syndrome. Risperidone is also used in low doses for treating autistic spectrum disorders. Risperidone's therapeutic activity in schizophrenia is believed to be mediated through a combination of dopamine Type 2 (D2) and serotonin Type 2 (5HT2) receptor antagonism.
Currently, the commercial sustained-release product of an atypical psychotic is RISPERDAL® CONSTA® marketed by Janssen, L. P. RISPERDAL® CONSTA® is an intramuscular microsphere formulation and is intended to deliver therapeutic levels of risperidone for two weeks. However, due to the inherent lag phase of most microsphere products, the patient is required to supplement the first 21 days of RISPERDAL® CONSTA® treatment with daily doses of risperidone. Approximately three weeks after a single intramuscular injection of RISPERDAL® CONSTA® and concurrent daily doses of oral risperidone, the microspheres release sufficient risperidone in the systemic circulation that the patient can discontinue supplementation with daily doses of the oral therapy.
The primary limitation of liposomes and microspheres used in sustained-release delivery systems is, typically, the limited amount of drug that can be entrapped in the dosage form. The amount of space available to entrap drug is restricted by the structure of the particulate. Further, the size of the injection is limited by the discomfort of the patient.
Other sustained-release delivery systems such as solid, biodegradable rods, or nondegradable reservoirs typically require surgical implantation. Furthermore, for the nondegradable delivery systems, a second surgical procedure is required to remove the empty reservoir.
There is a continuing need to develop products providing increased bioavailability of risperidone. In particular, there is a need to develop sustained release formulations of risperidone that do not suffer from low bioavailability, poor release kinetics, injection site toxicity, relatively large volume injections, and inconveniently short duration of release.